KabylakeOpenBoardPkg/KBLRvp3: Remove unused PCD 'PcdUseHpetTimer'

For KabylakeRvp3, it is using the HPET timer unconditionally. The PCD
'PcdUseHpetTimer' is not being used.

This commit will remove the usage of the above PCD in package DSC files.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>